What it is

For 34 beaches of Florianópolis, the sea at each one at the hour you choose — the waves at the shore and the wind — and, in six icons, what that sea means for surfing, sailing, paddling, swimming, fishing and boating; kept apart, what is decided and what is only forecast. Picking an activity only highlights the beaches good for it: the hour, the waves and the wind stay the same. Swell starts from the open sea at the island's eastern edge: ECMWF's and NOAA's forecast, read once a day (the 00 UTC run, every 3 hours), with its error measured by satellites. A wave model of the whole island, over the seafloor of the Brazilian Navy's chart 1902, carries each swell to each beach: it bends round the headlands, grows in the shallows, fades behind them and breaks.

What is decided

Only the wave limits, and always over the measured band. The open sea's band comes from the error the satellites measured at this edge (427 passes at 24 h lead, 2023–2026, the band claims 388 of 428); the one used is the union of ECMWF's band and NOAA's, which over the 15 test months it never saw covered 96.5% of 21,259 satellite measurements. The island model carries both edges of the band to each beach (the breaking height grows with the sea, so the beach's band is exact over the model). If even the low edge is over the limit: DANGER, decided. If the band straddles it: CAUTION, with how far it is from turning. If the whole band is under it: under the limit — which does not mean safe.

The arithmetic is redone in your browser with the same files that made the published day, and compared with it (a sha256 over every height and every decision). Each beach and hour's certificate can be downloaded and redone by anyone.

Activity	The rule
Surf	Breaking waves between 0.3 and 2.6 m, best with a long period and an offshore wind; above 2.6 m: too big (decided over the band).
Kite	Wind from 11 to 34 knots, side- or onshore, without gusts far above the mean; an offshore wind of 11 knots or more blows you out to sea (forecast warning).
SUP	Flat water: waves under 0.15–0.5 m and wind under 3–7 m/s; above 1.0 m breaking: heavy surf (decided); an offshore wind above 18 km/h blows you out to sea (forecast warning).
Swim	Waves under 0.35–1.1 m, little wind and little current; above 1.4 m breaking: danger (decided over the band); waves over 0.9 m arriving square to a sandy beach: rip current likely (rule of thumb). Swim near a lifeguard post.
Fish	Some sea (0.15–1.8 m) and wind under 6–10 m/s; above 2.0 m breaking: danger on the rocks (decided over the band).
Boat	Open sea under 1.2–2.2 m, wind under 7–12 m/s and little surf to launch through; open sea above 2.5 m: rough (decided over the measured band); wind above 13 m/s (25 knots): strong wind (forecast warning).

What is forecast

The wind (ECMWF at 10 m, on the grid over the island: its error at the beaches has not been measured yet), the score that gives each activity its icon (good, doable, poor), the longshore currents (from the waves' height and angle), the rip-current risk (a rule of thumb), the bays' chop (from the wind), the tide and the water temperature (Copernicus). Forecast warnings carry a dashed border; what is decided, a solid one and capitals.

Measured, neither forecast nor decision: how much each beach usually breaks, seen from space — in 190 cloud-free Sentinel-2 passes (2022–2026), the share of its surf zone that was white with foam. It says what the beach is like (wide and shallow foams more for the same sea), not what the day will be.

The beach model

64 cases (16 directions × 4 periods, 6 to 15 s) on a 30 m grid of the whole island: refraction, shoaling and shelter, linear. At each beach the height is read at probes from 1 to 9–13 m deep; the wave breaks where it reaches 0.55 times the depth, marching in from deep water (and the largest value at a surf-zone probe, where one stretch of the beach focuses the swell). In the original Swell the probes stopped at 4.5–6.5 m and 14 of the 21 open-sea beaches could never read more than 2.5 m whatever the sea did; the limit is now 4.9 to 7.2 m.

What each number rests on

The arithmetic is exact, and every number rests on premises checked to different rungs: validated (against the sea itself, held out of the calibration), compared (against a model that keeps more of the physics), verified (the code against an exact solution) and assumed (adopted from a source). A conclusion is as checked as its weakest premise.

validatedunverified
The open sea and its band covered 96.5% of 21,259 satellite passes it had not seen; it claims 9 in 10.
comparedunverified
The island model beside a Boussinesq model (Celeris; Madsen & Sørensen) at 12 beaches, 556 cases of a 1 m swell: r 0.44, the island 0.27 m higher on average; on swells of 11 s and longer, 1.19 times its value (median), from 0.39 to 3.34 by beach. Compared at those 12; assumed at the others.
assumed
Breaking the wave breaks at 0.55 times the depth (Thornton & Guza, 1982); no sandbars; inside 10 m the profile follows the chart's 10 m line.

The arithmetic is exact; the weakest premise is assumed, and neither model was verified against an exact solution of its own equations. Where the island reads less than the Boussinesq model — Pântano do Sul (0.39), Ingleses (0.43) and Galheta (0.99) — "under the limit" calls for more caution than the arithmetic shows.

Limits

The island model's own error at the shore has not been measured (a measurement at the beach is missing: a buoy or a camera). Today's sandbars are not in the seafloor; inside 10 m the profile is fitted to the chart's 10 m line. Guarda do Embaú lies 0.6 km from the model's edge and is refused: there the wave entering through the edge has not yet bent over the seafloor. A wind sea shorter than 6 s is read as the 6 s case; a swell longer than 15 s is read as 15 s and flagged on the beach's card. Only daylight hours count. Not for navigation.

Sources

Sea and wind: ECMWF open data (CC BY 4.0) and NOAA WAVEWATCH III / GFS-Wave (public domain). Measured error: satellite altimeters (NOAA RADS), Janela's calibrated bands at the island-edge point. Tide and water temperature: Generated using E.U. Copernicus Marine Service Information (doi:10.48670/moi-00016). Foam seen from space: contains modified Copernicus Sentinel data 2022–2026. Seafloor: Brazilian Navy chart 1902 (DHN/CHM; non-commercial use, not for navigation) and GMRT (CC BY 4.0). Map and places: © OpenStreetMap, Protomaps. Island model: the original Swell (frontier-apps), the same 64 cases. Code (MIT): apps/swell.